1. Field of the Invention
This invention relates generally to the fabrication of a giant magnetoresistive (GMR) magnetic field sensor for a magnetic read head. More particularly, it relates to such a sensor with a reference (pinned) layer whose magnetization varies in direction.
2. Description of the Related Art
The giant magnetoresistive (GMR) sensor of the “spin-valve” (SV) configuration is a multi-layered configuration of magnetic and non-magnetic layers which includes a magnetically free layer, whose magnetic moment is free to respond to external magnetic stimuli, separated by a non-magnetic layer from a magnetically “pinned” layer, called the reference layer, whose magnetic moment is fixed in direction.
Typically, the magnetic moment of a reference layer is most effectively pinned by forming the reference layer as a laminated three layer structure, called a synthetic ferrimagnet (SyF). The SyF is formed as two layers of ferromagnetic material separated by a non-magnetic spacer layer. The proper choice of spacer layer material, usually Ru or Rh and spacer layer thickness, makes it energetically favorable for the two ferromagnetic layers to couple to each other with oppositely directed, exchange coupled magnetic moments to form the synthetic ferrimagnet, SyF. The SyF is formed on a layer of antiferromagnetic material (AF), called a pinning layer, that unidirectionally pins the magnetic moment of the ferromagnetic layer that it is in contact with. The remaining ferromagnetic layer then maintains its magnetic moment in the opposite direction as a result of the exchange coupling promoted by the spacer layer. We will denote such an antiferromagnetically pinned tri-layered structure of anti-parallel magnetized ferromagnetic layers as AF/AP1/SL1/AP2/SL2/FL. Here. AF denotes the antiferromagnetic pinning layer. The SyF is the combination AP1/SL1/AP2, where Ap1and AP2 denote the anti-parallel ferromagnetic layers and SL1 is the spacer layer that couples AP1 and AP2. FL denotes the ferromagnetic free layer and SL2 is the second spacer layer, that separates the free layer from the SyF and prevents a strong magnetic coupling between them.
The motion of the free layer magnetic moment relative to the reference layer magnetic moment changes the resistance of the sensor so that a “sense” current passing through the layers produces measurable voltage variations across the sensor. In particular, it is the cosine of the angle between the free and pinned layer magnetizations that determines the sensor resistance and it is the variations of the angle that produce the response of the sensor.
As the size of free layers decrease dramatically, there is a problem of maintaining a stable domain structure that will not be overly affected by temperature variations and produce undesirable signal noise (Barkhausen noise). The random thermal fluctuations of free layer domain structure is associated with small edge domains that do not form closed loops (called uncompensated poles) and are, therefore, energetically easily moved by thermal energy of order kT. The common method of eliminating such edge variations is by means of longitudinal biasing layers, which are permanent (high coercivity) magnetic layers positioned at the lateral edges of the free layer to magnetostatically couple with the edge domains and, in effect, compensate them and make them energetically stable. The biasing layers, however, need not be permanent magnetic layers. Mack et al. (U.S. Pat. No. 6,469,878 B1) teaches a longitudinal biasing method that positions antiferromagnetic exchange tabs formed adjacent to outer edges of the free layer. Other approaches to stabilize the free layer include a method of Smith et al. (U.S. Pat. No. 6,473,279 B2) who teach an in-stack three layer combination that includes an auxiliary free layer proximate to the free layer and separated from the free layer by a non-magnetic spacer layer. A pinning layer is directly exchange coupled to the auxiliary layer and the auxiliary layer is exchange coupled across the spacer layer to the free layer.
Another approach to the problem of free layer biasing is to form the free layer of a superparamagnetic material that requires no biasing at all. Daughton et. Al. (U.S. Patent Application Publication: US 2004/0023065 A1) discloses a thin film based magnetic field sensor of a spin-dependent tunneling type in which the free layer is a thin film of a superparamagnetic substance.
As the informational area density of magnetic media read by such GMR sensors approaches and even exceeds 200 Gb/in2, the ability of such sensors to accurately read increasingly narrow tracks becomes critical. As recorded track widths decrease in width and as linear recorded density increases along the length of the track, the trackwidth and layer thickness of the GMR sensor free layer must both decrease. However, the sensitivity of free layers having such ultra-narrow trackwidths and thicknesses, which is the ability of the free layer magnetic moment to be rotated by the external fields of the recorded media, becomes worse as a result of the free layer edge demagnetization field and the longitudinal bias field from permanent magnets used to maintain a stable free layer domain structure. As noted above, as the free layers become thinner and narrower, the edge pole compensation provided by the longitudinal bias layers actually begins to dominate the magnetic fields of the recorded media and the free layer stability becomes a loss of sensitivity.
One way of improving the response of a GMR sensor that does not address the sensitivity of the free layer involves an improvement of the magnetic properties of the reference layer. In this regard, Lin et al. (U.S. Pat. No. 6,117,569), Lin et al. (U.S. Pat. No. 6,262,869 B1) and Lin et al. (U.S. Pat. No. 6,127,053), teaches, in varying configurations, an improved reference layer wherein a keeper layer formed on the reference layer generates a uniaxial stress-induced anisotropy in the reference layer that substantially increases the exchange pinning field with the antiferromagnetic pinning layer.
The present invention significantly improves the sensitivity of a GMR spin-valve configuration, not by an improved biasing scheme, but by effectively increasing the angle between the free and pinned layer magnetizations for a given external field produced by the magnetic medium. In the traditional spin-valve configuration it is only the magnetization of the free layer that moves in response to external magnetic stimuli. The pinned layer is a “reference” layer in the full sense of the word, it serves only to provide a fixed magnetization relative to which the magnetization of the free layer forms an angle that determines the resistance of the sensor. In the present invention, the magnetization of the reference layer is also allowed to move, so that the angular dependence of the relative magnetizations is enhanced and, along with it, the response of the sensor.
To produce a reference layer with variable magnetization, a novel configuration with three magnetic layers, denoted for now as P1, P2 and P3, separated by non-magnetic spacer layers, and pinned by an antiferromagnetic layer (AF) is required. Certain magnetic properties of a three layer configuration of the form: AF/P1/Ru/P2/Ru/P3 have been studied using calculational models and have been reported in “Analysis on Giant Magnetoresistive Characteristics of Synthetic Antiferromagnet-Based Spin Valves With Modified Pinned Layers” by Jeong-Suk Park, Seong-Rae Lee and Young Keun Kim, IEEE Transactions on Magnetics, Vo. 39, No. 5, September 2003, pp. 2399-2401. Of particular interest in this analysis are the exchange energies between AF-P1, P1-P2 and P2-P3.
The present invention provides a novel variation of the three layered configuration of Park et al. By careful control of the spacer layer material and thickness, the three layered configuration of the present invention allows two of the layers to be strongly coupled (high exchange magnetic field), while the magnetization of the third layer, being the layer adjacent to the free layer, is relatively free to free to move in the presence of the external field of the recorded medium. In this way, the three layer configuration plays a dual role, one pair of its layers providing a stable fixed reference direction, the third layer moving in a way that enhances the GMR effect of the sensor.